Tone control amplifier



June 29, 1948.

Filed Jan. 22, 1943 4 RI R1 MTG wuQ kn Nb A L W \QQ QM KW wt &

INVENTOR P/[RRE AIL. V/SSQHERS TTORNEY Patented June 29, 1948 werp,

Belgium, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application January 22, 1943, Serial No. 473,274 In-theNetherl'ands September 12,1941

This invention relates t tone controls and more particularly vtotone controls forradio receivers and the like. a V l A principal object isto provide an improved tone control for audio frequency amplifiers employing reversed or negative feedback. I

It has been proposed heretofore to provide the reversed feedback circuit of anaudio frequency amplifier with circuit connections for tone' control, whereby the tone control' and'negative feedback are combined. This prior combination controls the cutoff of the higher frequencies of the audio band whenthe tone control is turned; in one direction but does not exercise a like effect on the higher frequencies when turned in the opposite direction.

Accordingly, one" of the principal: objects of the present invention, is to provide atone control adjustment such that when itis turned in one direction it cuts off the higherfrequencies and when turned in the. opposite direction." it produces arising gain in thehigher' frequencies; The results obtainable with this arrangement are 'comparable'with'those obtainedrby other but more-costly and complicated means; e. g., variable-u selectivity controls, band-width controls and :the: like;

'I'he invention is characterized by a tone con trol comprising a potentiometer which' is connected symmetrically between and in' series I with a "16 the invention is concerned primarilywith the" tone control of the audiofre'quency ampli tier. of a radio receiver, 'itwill' be""u'nderstc'iodthat it is applicable to other typesofamplifiersfwhere higher andlowerfreduencies' of a'bandb'f signer frequencies arebeing' amplified." Furthermore; only those parts'of a two-stage audio 'freduney' amplifierare shown. as are sufficient to enable the inventive concept to be understoo It} will limited to a two-stage amplifier but can be embodied in more than two stages.

Accordingly, in the single figure of the drawing, there is shown a part of a radio receiving set, namely the audio frequency amplifier stages embodying the inventive features.

Referring to the drawing, there is shown a first amplifier stage VI comprising for example a triode of the indirectly heated cathode type,

. although as will be understood any other type of grid-controlled amplifiertube may be employed.

The audio frequency signals to be amplified are impressed in the usual manner on the control grid CGI of tube VI and the cathode K is returned to ground through the biassing combination of resistor BRI and parallel condenser BC! R2, to the input side of stage Vl. However, in

through a negative feedback'resistor RI. The source of heating current for the cathode heater H! is omitted from the drawing for the sake of simplicity. The secondlstage of the amplifier comprises for example a double grid tube V2 which may be a so called shield-grid tube comprising the indirectly heated cathode K2 with its heater filament H2, a control-grid CG2, a shieldgrid SG2 and the anode plate P2. The signal output circuit of tube VI is coupled for example known manner. Likewise, agrid leakresistor GL is connected tothe grid CGZ in the well-known manner. This grid isbiassed in the customary V2 is connected by thereverse d" feedback line L through "the usual reversed'feedback resistors RI,

addition to thesetwo conventional resistors; RI

and R2, another resistor R4 is, according to the linvention, connectedin'seriesin this negative. feedbackline. For the purpose of tone control (inaccordance with thepresent invention, the

grid'CGZ and the plate Pl are connected to a point between the resistors R2 and R4 through arm SD of this potentiometer is returned to g'round'as shown.

The condenser C l and the potentiometer resistance R3 are therefore similar to those combeunderstood of'coursethattheinventioiiis'notf t5 monlyusedffortonejcontrol. Theaction of these The output of the two elements Cl and R3 in the usual tone control circuit is that when the slider SL of the potentiometer is moved in one or the other direction, the effective loading of the plate circuit of tube VI is increased or decreased for the higher audio frequencies. In other words, when the slider SL is moved so as to connect the condenser Cl directly to ground, the external plate impedance of tube VI decreases as the signal frequencies increase, because of the action of condensers Cl which is then connected from plate to ground. When the slider SL is moved in the opposite direction, that is towards condenser C2, there will be a decrease in the plate loading of tube V! for the higher frequencies proportionally to the increase in the resistance which is now included in series with Cl and ground. The effect of this therefore, is to reduce the cutoff effect at the higher audio frequencies. By connecting the tone control circuit in the manner shown in the drawing with a suitable proportion of values of condenser C2, resistances R2 and Bi, an additional advantage is obtained. While the usual reversed feedback circuit consists of the two-resistances RI and-R2, when the amplification of the two stages V1 and V2 is normal, the reversed feedback compensation therefor, will be approximately uniform over the entire frequency band to be covered. By the addition of R6 and'C2 in combination with R3, however, this action can be modified according to the position of the slider SL and the selected values of R4 and C2. These two values are selected in accordance with the maximum action obtainable from the circuit shown, where R3 because of the fact that it is variable permits the modification of the action up to the maximum obtainable by means of the selected Values of C2 and R4. The action of C2 and R6 when'the slider 'SL is in a position where the said slider is closer to C2 than to Cl -is as follows. "As the signal frequency increases, the impedance of C2 decreases in proportion thereto and will therefore, in combination with Rd, produce a voltage drop proportional to the frequency increase across R4 of the voltage derived from the line L and which, through R4 and R3, is applied to the cathode of tube Vl across RI. This also reduces the amount of reversed feedback in proportion to the increase in the frequencies, and therefore increases the overall amplification of the stage Vl for these frequencies.

Depending upon the position of the slider SL, this action will be maximum when there is no resistance left between the slider and C2, or will progressively decrease as the slider is moved in the other direction and the series resistance between the slider and C2 increases. This, therefore, constitutes a means of adjusting the amount of the action of this part of the circuit. The maximum obtainable depends upon the amount of reversed feedback used.

Considering the complete working of the circuits combination it follows that by moving the slider to the side of the potentiometer connected to Cl that higher frequencies are cut-off by the action of Cl and, therefore, will cause reduction in the reproduction of the higher audio-frequencies. By moving the slider of the potentiometer R3 to the other side,this cut-off action is first reduced, then, when the amount of resistance is large enough, will cease entirely. By moving the slider further to the side connected to C2 we have the reversed action, i. e., rising for the higher frequencies, which action will increase when the amount of resistance between the slider and C2 decreases to the minimum obtainable which'is,

when the slider is in direct contact with the point of the potentiometer connected to C2, and this maximum action will depend upon the values of C2 and R4 and the amount of reversed feedback used in total.

Various changes and modifications may be made in the disclosed embodiment without departing from the spirit and scope of the invention.

What I claim is:

1. An amplifier including at least one gridcontrolled audio frequency amplifier tube, a negative feedback circuit for said tube, a tone control including a pair of condensers in series between the plate of said tube and the negative feedback circuit, adjustable means movable in one direction to increase a'resistance in series between one of said condensers and ground while decreasing the amount of resistance between the other condenser and ground, said tone control being connected to said negative feedback circuit whereby adjustment of said tone control simultaneously achieves the effect of variable selectivity. v

2. A plural stage audio frequency amplifier comprising a first amplifier tube and a second amplifier tube coupled together for signal amplification, a negative feedback resistor connected to the input circuit of the'first tube, means to apply a voltage to said resistor under control of the output of said second tube including a pair of resistors connecting the output of the second tube to the input of the first tube, a tone control circuit in the form of a T-branch connected from a point between said pair of resistors to a point in the output circuit of said first amplifier tube, said tone control circuit including in series a potentiometer resistance and a condenser connected to each end of said potentiometer resistance, the slider of said otentiometer resistance being connected to ground.

3. An amplifier including at least one electron discharge tube, a reverse feedback circuit and a shunt path connected at one end to the anode of one said tube and at the other end to a lead of the reverse feedback circuit and including a condenser connected as a plate load, a second condenser connected to said lead of the reverse feedback circuit, a potentiometric resistance connected in series between said condensers and an adjustable arm' directly connected to ground, the movement of said arm along said potentiometric resistance simultaneously effecting tone control and varying the frequency characteristic of the reverse feedback current.

4. An amplifier as set forth in claim 3 in which a resistance is inserted in said reverse feedback circuit between the point of tapping the reverse feedback current and the point at'which said second condenser is connected to said reverse feedback lead, the values of said resistance and of said condenser and .the position of said adjustable arm determining the characteristics of the reverse feedback current. V

5. An amplifier as set forth in claim 3 in which said reverse feedback circuit includes a pair of series resistances and said second condenser is connected to said .reverse feedback lead at a point between said resistances.

6. In an amplifier system, a frequency-discriminating amplification control arrangement,

, frequency-discriminating means in' said feedback circuit means forreducing the reverse feedback in said predetermined band of frequencies to increase the amplification thereof, and variable circuit means forming part of said tone control means and of said frequency-discriminating means for increasing the amount of attenuation by said tone control means of said predetermined band of frequencies when varied in one direction and for reducing the reverse feedback through said frequency-discriminating means to increase the amplification of said band of frequencies when varied in the opposite direction, whereby the amplified signal in said predetermined range of frequencies may be varied from a condition of considerable attenuation when said variable means is at one end of its control range to a condition of considerably more than normal amplification when said variable means is at the other end of its control range.

'7. In an amplifier system, a, frequency-discriminating amplification control arrangement, comprising, in combination, tone control means for attenuating a predetermined band of frequencies in the signal being amplified comprising a condenser and a resistor in series shunting the signal path in the amplifier system, reverse feedback circuit means for said amplifier system, and a frequency discriminating circuit comprising said resistor and a second condenser in series shunted across said feedback circuit means for reducing the reverse feedback in said predetermined band of frequencies to increase the amplification thereof, said resistor being variable to increase the amount of attenuation by said tone control means of said predetermined band of frequencies when varied in one direction and to reduce the reverse feedback through said frequency-discriminating circuit to increase the amplification of said band of frequencies when varied in the opposite direction, whereby the amplified signal in said predetermined range of frequencies may be varied from a condition of considerable attenuation when said variable resistor is at one end of its control range to a condition of considerably more than normal amplification when said variable resistor is at the other end of its control range.

8. In an amplifier system including an electron discharge tube, a frequency-discriminating amplification control arrangement, comprising, in combination, a tone-control circuit shunting the tube plate circuit to ground for attenuating a predetermined band of frequencies inthe signal being amplified comprising a first condenser and resistor connected in series and shunting the signal path in the amplifying system, reverse feedback circut means for said amplifier system, and frequency-discriminating means in said feedback circuit means and including said resistor and a second condenser connected in series and shunting said feedback circuit for reducing the reverse feedback in said predetermined band of frequencies to increase the amplification thereof, said resistor bein a potentiometer with its variable contact connected to ground for increasing the amount of attenuation by said tone control means of said predetermined band of frequencies when varied in one direction and for reducing the reverse feedback through said frequency-discriminating means to increase the amplification of said band of frequencies when varied in the opposite direction, whereby the amplified signal in said predetermined range of frequencies may be varied from a condition of considerable attenuation when said potentiometer is at one end of its control range to a condition of considerably more than normal amplification when said potentiometer is at the other end of its control range.

9. An amplifier system as described in claim 8 wherein said resistor is a potentiometer having a sliding contact connected to ground and said first condenser is connected between one end of said potentiometer and the plate of said tube, and said second condenser is connected between the other end'of said potentiometer and an intermediate point in said feedback circuit, said intermediate point in said feedback circuit constituting the juncture between two resistors connected in series from the output of the amplifier system to ground, the feedback voltage being developed across the grounded resistor.

PIERRE AUGUST JOSEPH LEOPOLD VISSCHERS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,101,688 Rechnitzer Dec. 7, 1937 2,105,558 Smith Jan. 18, 1938 2,282,870 Lundie May 12, 1942 2,317,025 Bond Apr. 20, 1943 

